1. Field of the Invention
The present invention relates to wireless RF communications systems and related methods. More particularly, the present invention relates to wireless RF communications systems and related methods between cellular base stations and user equipment.
2. Description of the Prior Art and Related Background Information
Achieving high data transfer rates with good signal quality is a primary goal of modern wireless communications systems. Orthogonal Frequency Division Multiplex (OFDM—also referred to as Orthogonal Frequency Division Multiple access) modulation is an approach which uses the parallel transmission of multiple orthogonal sub-carriers to achieve high data transfer rates. One current example of a global wireless communications standard using OFDM is the LTE (Long Term Evolution) standard. The down-link waveform used in LTE is based on an OFDM format which, like most OFDM waveforms, can have a large peak-to-average power ratio (PAPR).
As noted above, the LTE waveform can have a high peak to average power ratio. Even though all modulation symbols assigned to the sub-carriers have the same average power, the magnitude of the down-link LTE waveform varies significantly in the time domain. This is due to the IFFT operation that forms each time sample from a sum of random phase variables. Phase alignment of sub-carriers results in large peaks in the time domain. Large peaks cause problems because the power amplifiers become less efficient as the peak-to-average power ratio (PAPR) of the RF signal increases. In addition, the finite dynamic range of digital-to-analog converters (DACs) place limits on the PAPR. As a result, it is desirable to limit the PAPR to allow for a more efficient design of the transmitter. This process is referred to as crest factor reduction (CFR). Usually crest factor reduction introduces in-band errors that increase the error vector magnitude (EVM) and bit error rate (BER) of the demodulated signal at the receiver.
CFR can be accomplished in various manners. The direct method is to clip peaks exceeding a specified level. This has the effect of moving the complex-valued symbols from their assigned constellation positions. The difference between the actual and assigned position in the IQ space is referred to as the “constellation error” and is measured using the error vector magnitude (EVM). Clipping tends to distribute the constellation error over the all the available sub-carriers. This can be problematic for LTE if the P-SCH, S-SCH, and reference signals are degraded. High order constellations, such as 64QAM, are especially sensitive to EVM and therefore this technique has very limited results.
Another class of methods for CFR uses some of the sub-carriers as peak reducers. This includes “tone reservation.” Once a peak is detected in the time domain, the phases of the reserved sub-carriers are selected to reduce the peak. This results in lost bandwidth because fewer sub-carriers are available from transmitting data. Information regarding the active data sub-carriers must be sent to the receiver.
Another method for CFR is to alter the constellation so that the mapping between data bits and Complex-valued modulation symbols is not unique. This is referred to as “constellation extension.” Data bits are mapped to two constellation positions so that cIQ=−cIQ where cIQ is the complex-valued modulation symbol. The downside of this approach is that one bit is lost in the constellation mapping which reduces the throughput for tile QPSK, 16-QAM, and 64-QAM to ½, ¾, and ⅚ of the original value, respectively.
Other methods for CFR attempt to randomize the phase of the sub-carriers. These include “partial transmit sequence” (PTS) and “selective mapping” (SLM). The sub-carriers are multiplied by a set of different phase vectors producing a set of potential time sequences. The time sequence with the lowest PAPR is transmitted. The disadvantage of such approaches, typically, is that the information regarding the phase vector used must be sent to the receiver to allow demodulation.